ostree/config: Add an "unset" operation

Currently there's a way to set a key to the empty string but there's no
way to unset it completely (remove the key from the group). This might
be helpful for instance if you want to temporarily set
"core.lock-timeout-secs" to a specific value for the duration of one
operation and then return it to the default after that operation
completes.

This commit implements an "unset" operation for the config command, adds
a unit test, and updates the man page.

Closes: #1743
Approved by: cgwalters

man/ostree-config.xml

@@ -62,14 +62,37 @@ Boston, MA 02111-1307, USA.
             <cmdsynopsis>
                 <command>ostree config set</command> <arg choice="req"> --group=GROUPNAME</arg> <arg choice="req"> KEYNAME</arg> <arg choice="req">VALUE</arg>
             </cmdsynopsis>
+            <cmdsynopsis>
+                <command>ostree config unset</command> <arg choice="req">SECTIONNAME.KEYNAME</arg>
+            </cmdsynopsis>
+            <cmdsynopsis>
+                <command>ostree config unset</command> <arg choice="req"> --group=GROUPNAME</arg> <arg choice="req"> KEYNAME</arg>
+            </cmdsynopsis>
     </refsynopsisdiv>
 
     <refsect1>
         <title>Description</title>
 
-        <para>
-            Displays or changes a configuration setting.
-        </para>
+        <itemizedlist>
+            <listitem><para>
+                <command>ostree config get</command> displays the value of
+                <arg choice="plain">KEYNAME</arg> in the group <arg choice="plain">GROUPNAME</arg>
+                (or <arg choice="plain">SECTIONNAME</arg> depending on the
+                syntax used).
+            </para></listitem>
+            <listitem><para>
+                <command>ostree config set</command> sets the value of
+                <arg choice="plain">KEYNAME</arg> in the group <arg choice="plain">GROUPNAME</arg>
+                to <arg choice="plain">VALUE</arg>.
+            </para></listitem>
+            <listitem><para>
+                <command>ostree config unset</command> removes the key
+                <arg choice="plain">KEYNAME</arg> from the group <arg choice="plain">GROUPNAME</arg>
+                so that OSTree uses the default value for it. It is not an
+                error for the specified <arg choice="plain">GROUPNAME</arg> or
+                <arg choice="plain">KEYNAME</arg> not to exist.
+            </para></listitem>
+        </itemizedlist>
     </refsect1>
 
     <refsect1>
@@ -77,5 +100,6 @@ Boston, MA 02111-1307, USA.
         <para><command>$ ostree config get core.mode</command></para>
         <para>bare</para>
         <para><command>$ ostree config set --group='remote "myremote"' url http://example.com/repo</command></para>
+        <para><command>$ ostree config unset core.lock-timeout-secs</command></para>
     </refsect1>
 </refentry>